PROCEEDINGS, 42nd Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, February 13-15, 2017

SGP-TR-212

1

Geochemical Evaluation of the Geothermal Resources of Camas Prairie, Idaho

Ghanashyam Neupane1,2,*

, Earl D. Mattson1, Nicolas Spycher

3, Patrick F. Dobson

3, Mark E. Conrad

3, Dennis L. Newell

5,

Travis L. Mcling1,2

, Thomas R. Wood2,4

, Cody J. Cannon6, Trevor A Atkinson

7, Corey W. Brazell

1,2, and Wade C.

Worthing2,4

1Idaho National Laboratory, Idaho Falls, ID 83415, USA

2Center for Advanced Energy Studies, Idaho Falls, ID 83401, USA

3Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

4University of Idaho-Idaho Falls, Idaho Falls, ID 83402, USA

5Utah State University, Logan, UT 84322, USA

6Geosyntec Consultants, Tampa, FL 33637, USA

7DOSECC Exploration Services LLC, Salt Lake City, UT 84104 USA

*Email: Ghanashyam.Neupane@inl.gov

Keywords: Snake River Plain, geothermometry, Camas Prairie, Idaho batholith

ABSTRACT

The Camas Prairie in Blaine, Camas, and Elmore Counties, Idaho has long been identified as an area with geothermal potential, and is

one of several promising geothermal prospects that were identified by play fairway analysis of the Snake River Plain region. The

geothermal activity in the area is manifested by the presence of several hot springs. Moreover, a number of groundwater wells with

elevated temperatures have been identified in the area. However, the location of geothermal resources within the basin is poorly

constrained, as they are concealed by Quaternary and Neogene sedimentary deposits and volcanic rocks. The prairie is likely formed by

failed rifting or extensional faulting as a response to the down warping of the central Snake River Plain to the south. Young volcanic

activity and faulting are evident within the prairie and along its margins. As a part of a more comprehensive effort to assess the

geothermal resource potential of the area, we present new water chemical and isotope results from the area in the context of existing

water chemistry data. We applied conventional and multicomponent geothermometry tools to estimate the reservoir temperatures for the

area. Specifically, our geochemical and geothermometry analysis indicated two interesting areas, one in the central-west northern side

and other in the central-west southern side of prairie. Our results show that the Camas Prairie area could potentially host resources with

temperatures as high as 200 ºC. Water chemical and isotopic results suggest that the highest temperature resource is a mixture between

deeply circulated fluids with a magmatic affinity and local meteoric recharge. The water geochemistry suggests that there are two

potential geothermal resource types: one associated with the Idaho Batholith to the north, and a second related to elevated heat flow

associated with Quaternary volcanism and intrusions within the Snake River Plain province. Ongoing geophysical, geological, and

geochemical investigation of the area could be instrumental in defining the Camas Prairie geothermal resource.

1. INTRODUCTION

Camas Prairie is an east-west elongated (about 50 km by 15 km) valley in Blaine, Camas, and Elmore Counties in south-central Idaho

(Figure 1). In general, the prairie is bounded by the Mount Bennett Hills to the south and the Soldier Mountains to the north. The Mount

Bennett Hills represents an east-west trending horst that runs parallel to the Camas Prairie. On the south side of Mount Bennett Hills lies

the central parts of the Snake River Plain, which is a magmatic region of high heat flow with great potential for significant geothermal

resources. Similarly, the Soldier Mountains to the north of Camas Prairie represents a large area of central Idaho with faulted mountains

associated with Cretaceous-Miocene magmatic/volcanic activity.

The Snake River Plain is a topographic depression along the Snake River in southern Idaho. The western Snake River Plain is a basalt-

and sediment-filled tectonic feature defined by a normal fault-bounded graben whereas the eastern Snake River Plain is formed by

crustal down-warping, faulting, and successive caldera formation that is linked to middle Miocene to Recent volcanic activity associated

with the relative movement of the Yellowstone Hot Spot (Pierce and Morgan, 1992; Hughes et al., 1999). The genesis of the Mount

Bennett Hills along with Camas Prairie have been associated with the evolution of the Snake River Plain over time (Cluer and Cluer,

1986).

The occurrence of several clusters of hot springs within and along the margins of the prairie suggest the presence of geothermal

resources in the Camas Prairie (Mitchell, 1976). Specifically, five local areas with hot-spring activity are identified as having

geothermal potential. The Magic Hot Springs and the Elk Creek Hot Springs areas are located in eastern and northeastern sides of

Camas Prairie, whereas the Sheep and Wolf Hot Springs (~50 ºC) are located in western part of Camas Prairie. Another area with

geothermal potential is associated with the Wardrop Hot Springs (60 °C) located in the north-central part of the prairie near the base of

the Soldier Mountains. The fifth hot springs area in Camas Prairie is represented by the Barron Hot Springs (73 °C) area located on the

southern side of the prairie near the base of the Mount Bennett Hills. Several hot wells are also located in the vicinity of these hot

springs. With the exception of residential space heating applications, the geothermal resources of the area are not utilized for any

commercial activities.

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Since 2014, we have been conducting field studies in this area as a part of geochemical, isotopic, and geothermometric evaluations of

the geothermal resources in and around the Snake River Plain in southern Idaho (e.g., Neupane et al., 2014; Cannon et al., 2014; Dobson

et al., 2015; Neupane et al., 2016; Conrad et al., 2016). This work represents the continuation of our work in southern Idaho with a fresh

impetus for the Camas Prairie area as one of the Snake River Plain Play Fairway Phase II focus areas (Shervais et al., 2016). In this

paper, we present geochemical, isotopic, and geothermometric results based on recent as well as previously published

geochemical/isotopic data (e.g, Mitchell, 1976).

Figure 1. Maps of (a) Idaho with an ellipse indicating the general location of Camas Prairie, (b) southern Idaho showing the

general distribution of sampling features in the Camas Prairie area, and (c) Camas Prairie area showing locations of

various sampled sites - Magic Hot Springs Reservoir Landing well (RLW): red push-pin; Wardrop/Sheep/Wolf Hot

springs: red circles; Elk Creek Hot Springs: purple circles; Barron Hot Springs (and associated samples): yellow circles;

cooler groundwater wells and cold springs: blue circles.

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2. GEOLOGIC AND GEOTHERMAL SETTINGS

Camas Prairie is an intermontane valley filled with sediments and basalt layers over granites/granodiorites of the Cretaceous Idaho

Batholith (Garwood et al., 2014). Mitchell (1976) considered this area as a simple depression in the granitic surface of the Idaho

Batholith that has been filled in with valley alluvium and colluvium from the hills/mountains on both sides. This structural setting was

thought to be supported by outcrops of granite on the northern and southern borders and granite found at a shallow depth beneath

alluvium in the center of the prairie (Mitchell, 1976). However, previously, Smith (1966) referred the basin as a fault-bounded graben.

Cluer and Cluer (1986) compiled a suite of geologic and topographic evidence and suggested that the prairie is a result of failed rifting

of the area controlled by north-south extensional tectonics. They made an argument for a “Camas Prairie Rift” which was believed to

have occurred between 2-5 Ma and lasted for a relatively short time. The loading and down-warping of the Snake River Plain to the

south created an extensional regime along the Camas Prairie region that created marginal faulting and development of a rift valley

separating the Snake River Plain from the Idaho Batholith region. Subsequently, basalt and sediment layers filled in the rift valley and

shaped the present day Camas Prairie (Figure 2). Although the schematic diagram (Figure 2) shows almost a kilometer of valley-fill

sediments at the center of Camas Prairie, preliminary results of the ongoing Snake River Play Fairway Phase II project indicate that the

valley-fill sediments may be much thinner. Concerted efforts combining seismic, electromagnetic, and gravity surveys (Glen et al.,

2017) will help define the structural setting of this area in the near future. In general, faults that parallel the Snake River Plain with

opposite senses of displacement are present in the Mount Bennett Hills and along the edge of the Soldier Mountains. One of these faults

is inferred to be continuous but concealed through the prairie in close proximity to the Sheep/Wolf Hot Springs area. The spring waters

have likely migrated upward from a deeply buried fracture zone in the granodiorite along this fault. The water could also be under slight

artesian pressure because most of the Camas Prairie is below the regional potentiometric surface (Mitchell, 1976).

Figure 2. Schematic geologic cross-section of Camas Prairie (after Cluer and Cluer, 1986).

The Mount Bennett Hills to the south of the prairie are composed predominantly of Miocene rhyolitic ash flows and lava flows of the

Idavada Volcanic Group (Tfv) that overlies the Idaho Batholith granodiorite (Kg). Local basalt flows and fluvial/lacustrine sediments

are also present. The Soldier Mountains are composed mostly of Cretaceous granodiorite (Kg) with minor amounts of younger

intrusives. Camas Prairie is host to an unknown thickness of Quaternary alluvial, fluvial, and lacustrine sediments (Qs) with local

lenses of basalt (Qb) encountered in the shallow subsurface (Figure 2).

The Elk Creek Hot Springs area (Figure 1) is located in the southernmost regions of Soldier Mountains near the Elk Creek drainage, a

few kilometers north from the Camas Prairie basin. Rocks in the area include - Miocene Tuff of the Cannonball Mountain Formation

(Tcm) of the Idavada Group, Eocene dacite and rhyodacite of the Challis Volcanic Group (Tcvd), a diorite and gabbro unit (Tdg), and

the Cretaceous Idaho Batholith granodiorite (Kg) (Garwood et al., 2014). Various Tertiary dacite and rhyolite dikes (Td) are mapped

throughout the area intruding into batholith granodiorites, and based on local abundance, are inferred in cross-section (Figure 3).

Alluvial fan deposits are restricted to slopes and valleys.

Many NW trending normal faults with orientations similar to Basin and Range extensional structures dominate. Locally, the SW

dipping, NW trending Elk Creek Fault extends from the Camas Prairie north into the Soldier Mountains. This fault and associated

breccia facilitate flow from deeply jointed and fractured granodiorite (Figure 3). Also, the presence of numerous dikes in the area

represents planes of weakness within the batholith that could serve as preferential pathways for geothermal fluids.

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Figure 3. Geologic cross-section through the Elk Creek Hot Springs area (based on Garwood et al., 2014).

The Magic Hot Springs area (Figure 1) lies at the eastern end of the prairie. The Magic Hot Springs area consists predominantly of

Miocene-Quaternary silicic volcanic rocks and basalt flows. The Pliocene-Miocene Poison Creek Tuff (Tpct) is the uppermost unit in

the immediate vicinity of Magic Hot Springs Reservoir and is underlain by the Miocene Tuff of City of Rocks (Tcort), a rhyolite tuff

from the Idavada Group (Figure 4). Other rhyolites and basalt flows are abundant in the surrounding areas but are not shown in the

cross-section. The Cretaceous Idaho Batholith rocks (Kg) form the basement throughout the region.

Figure 4. Geologic cross-section of the Magic Reservoir Hot Springs area

Structurally, this area is interesting because of its location at the intersection of several regional to local geographic and geologic

features including the Eastern Snake River Plain, Camas Prairie, eastern part of Mt. Bennett Hills, and Idaho Batholith. The Magic Hot

Springs area is in a tensional stress regime and includes many high-angle normal faults that create block-faulted configurations

(Struhsacker, 1982). Mitchell (1976) recognized two curvilinear features from Landsat false color infrared satellite imagery and

discusses their controlling nature in the immediate area. The Magic Hot Springs Reservoir Fault trends northwest and extends the length

of the reservoir and into the northern Soldier Mountains. Another fault extends at a slightly less northwest trend along the Clay Bank

Hills and intersects the Magic Hot Springs Reservoir fault near the location of the Magic Hot Springs Reservoir Landing well (RLW)

(Malde et al., 1963). Struhsacker et al. (1982) refer to the resulting structure as the Hot Springs Landing horst. These structures are

interpreted to have occurred prior to Quaternary volcanism due to the lack of deformation in the flat lying young basalts and sediments.

They may also be related to a buried caldera inferred from stratigraphic thicknesses and basalt vent locations in the surrounding regions

(Leeman, 1982).

3. GEOCHEMICAL TOOLS

3.1 Solute Geothermometry

One of the tools applied by our group for geothermal prospection is solute geothermometry, which entails estimating deep reservoir

temperatures from the chemical compositions of water from springs and wells. As an exploration tool, geothermometry offers a cost

effective method to decrease exploration risk by evaluating a potential geothermal reservoir’s temperature. To conduct geothermometry,

measured chemical compositions of water from wells and springs are needed. The application of geothermometry requires several

assumptions. The most important assumptions are that the reservoir minerals and fluids at depth are at or near chemical equilibrium, and

that the chemical signature of this equilibrium is preserved as the water moves from the reservoir to the sampled location (Fournier et

al., 1974).

Traditional (i.e., quartz, chalcedony, Na-K-Ca, Na-K-Mg trilinear,) and multicomponent (RTEst and GeoT) geothermometers were used

to assess the reservoir temperatures of the geothermal systems in Camas Prairie area. Details about these tools can be found elsewhere

(e.g., Fournier and Truesdell, 1973; Fournier, 1977; Fournier and Potter, 1979; Giggenbach, 1988; Palmer et al., 2014; Spycher et al.,

2014).

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3.2 Isotopes and isotopic geothermometry

Another independent geothermometric approach carried out for this study was to compare the isotopic compositions of different

components of the fluids to calculate the temperature at which the two components would have been in isotopic equilibrium (e.g., the

oxygen isotopic composition of dissolved sulfate and the water or the carbon isotopic composition of dissolved inorganic carbon and

methane). For this work, we used sulfate-water oxygen isotope values to estimate reservoir temperatures (Fowler et al., 2013) and

compared the isotopic temperature estimates with temperature estimates obtained with geochemical geothermometers.

We also measured hydrogen isotope (deuterium) values in some water samples from this area. It is known that the hydrogen and oxygen

isotopic compositions of meteoric water are generally related to each other in a systematic way (Craig, 1961), that waters of distinct

isotopic compositions can represent distinct sources (Giggenbach, 1992), and that the water interactions with rocks at high temperatures

can shift the oxygen isotopic composition of the water towards equilibrium with the rocks with little effect on the hydrogen isotopic

composition of the water (Taylor, 1974). In this study, the distinct isotope exchange behavior of oxygen and deuterium was used to infer

high temperature interaction between the water and rocks. As a concerted geochemical/isotopic evaluation of geothermal resources in

the Camas Prairie (and broader southern Idaho areas, Dobson et al., 2015), we also measured He isotopes in water/gas samples from

various features and used them to identify the source of the He and associated fluids (Ballentine et al., 2002; Graham, 2002; Dobson et

al., 2015).

4. WATER CHEMISTRY RESULTS

4.1 Water compositions

Water samples from numerous geothermal features such as hot springs and wells with elevated temperatures were collected, and their

chemical constituents were analyzed in 2014-2016. Furthermore, existing water composition data were also assembled from the

literature (e.g., Mitchell, 1976; NWIS data base, etc.). The water sampling locations are shown in Figure 1. The highest field

temperature was recorded at the Magic Hot Spring RLW (75 °C). The pH of water samples ranged from 6.7 (Monument Gulch Spring, a

cold spring) to 9.9 (Sheep Hot Spring). These thermal waters show a large range in total dissolved solids (TDS) from about 120 mg/L

(unnamed spring #4, a cold spring) to about 1500 mg/L (Magic Hot Spring RLW).

Figure 5. Piper diagram representing chemistry of water samples from Camas Prairie area. Samples are grouped as

groundwater/cold springs (blue diamonds), Barron Hot Springs area (BHS, cyan triangles; BW1 indicates recent (2014

and 2016) water chemistry data of high sulfate water samples of the Barron Well 1), Sheep/Wolf Hot Springs (SHS, black

hourglasses), Wardrop Hot Springs area (WHS, red circles), Elk Creek Hot Springs (ECHS, magenta stars), and Magic

Hot Springs area (MHS, green squares).

The concentrations of major anions and cations in the groundwater and cold springs samples as well as in the water samples collected

from hot springs and thermal (with T>18 ºC) wells are illustrated in Figure 5. In most of the groundwater/cold springs samples, Ca and

HCO3 are the dominant cation and anion species, respectively (Ca-HCO3 type water). In general, groundwater/cold springs water

samples have near-neutral pH, low levels of F, Na (except in four wells and one spring that have relatively higher amounts of Na),

typically higher Mg concentrations than thermal waters, and a wide range of Ca concentrations. All thermal water samples have Na as

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the dominant cation and HCO3 as the dominant anion species, except in samples from the Barron Well 1 where SO4 is the dominant

anion.

In general, the aqueous chemistry of thermal water samples of Camas Prairie indicates that at least four groups of waters with distinct

properties are found in the area. The samples from the easternmost parts of the Prairie, the Magic Hot Springs area (all samples from

this area are from the Magic Hot Springs RLW), have relatively higher levels of TDS and silica, near neutral pH, and higher levels of

Na and HCO3 with significant amounts of Cl and SO4. Two samples from the Elk Creek Hot Springs area are alkaline (pH>9), high-F

(15 mg/L), Na-HCO3 type waters with very low Mg but significant Cl and SO4. Generally, alkaline pH and high F concentration are

used as distinct chemical characteristics of waters that interact with the Idaho batholith. The F-bearing accessory minerals in the

granite/granodiorite of the Idaho batholith are thought to be the source of high F content in these waters (Mitchell, 1976). The third

distinct group of water samples represents Sheep/Wolf Hot Springs and Wardrop Hot Springs area samples. Water samples from both of

these hot springs areas have high pH (9.0-9.9), relatively low level of TDS (220-350 mg/L), low concentrations of both Ca and Mg, and

intermediate concentration of F (1.9-3.7 mg/L). Finally, the last group of Camas Prairie water samples is from the southern part of the

basin, notably from the Barron Hot Springs area. These samples are near-neutral to slightly alkaline (pH 7.4-8.5) with slightly higher

TDS (380-640 mg/L), low concentrations of Mg, and moderately high concentrations of F (7-13 mg/L). Samples from Barron Well 1

from this area have unusually high SO4 (170-210 mg/L). This well is also noted for its large variations in measured temperature. For

example, we recorded a temperature of 38 ºC for this well in March, 2014; however, the same well was measured at 23 ºC in August,

2016. Another unusual aspect of waters from this well is the silica concentrations, which was measured at 52 mg/L in 2014 and 15 mg/L

in 2016. This well (and its neighbors) is near the no longer active Barron Hot Spring – perturbation of the local water table due to

extensive groundwater withdrawal for irrigation may be in part responsible for this hot spring’s demise. These spatial and temporal

variations in chemistry and T indicate that this well may be a less reliable feature for resource assessment, and may be affected by

seasonal pumping/irrigation activities. Therefore, this well is excluded from the solute geothermometric temperature estimation for this

area. Groundwater/cold springs water samples have near-neutral pH, low levels of F, Na, and wide range of Ca.

4.2 Isotopic compositions

The δD and δ18O of some Camas Prairie area samples are plotted in Figure 6. Most of the samples plot close to the local meteoric water

line (Benjamin et al., 2004); however, a few samples have oxygen isotope composition shifted 1-3‰ to the right of the meteoric water

line (e.g., Barron Well 1, Elk Creek Hot Spring 2, Magic Hot Springs RLW). These isotopic compositions, in general, indicate that the

groundwater/hydrothermal waters in the area are of meteoric origin.

Figure 6. Hydrogen and oxygen stable isotope ratios (‰ vs. SMOW) of Camas Prairie water samples with the global (GMWL;

Craig, 1961) and local (LMWL; Benjamin et al., 2004) meteoric water lines. Most of the samples show a small-degree of

shift to the right whereas a few water samples [e.g., Magic Hot Springs RLW and its runoff (MHS), Barron Well 1

(BW1), Elk Creek Hot Spring 2 (ECHS)] show a significant shift to the right (towards higher 18O values) of the meteoric

water lines, which suggests oxygen isotope exchange during high-temperature water-rock interaction in hydrothermal

systems, or could also indicate mixing with magmatically derived fluids. Other abbreviations: WHS: Wardrop Hot

Spring; SHS: Sheep/Wolf Hot Springs; BHS: Barron Hot Springs area samples, GW: Groundwater/cold spring samples;

HW: Higg’s well; BHW: Barron House well; GBW: Grandpa Barron well; GTW: Gonzales Thermal well; SPW:

Swimming Pool well; SW2015: Strom well 2015; SVR4: Sun Valley Ranch well 4; SVR5: Sun Valley Ranch well 5.

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5. GEOTHERMOMETRY RESULTS

5.1 Traditional geothermometry

Figure 7 shows that samples from the Wardrop/Sheep/Wolf Hot Springs, Elk Creek Hot Springs, and Magic Hot Springs are mostly

partially equilibrated thermal waters (Giggenbach, 1988). However, the majority of samples from the Barron Hot Springs area are

immature samples, with a few (about 6) samples plotting on the partial equilibration field. The Giggenbach plot for the Magic Hot

Springs area samples may indicate deep temperatures up to about 200 ºC. Similarly, the Wardrop/Sheep/Wolf Hot Springs samples may

indicate water-rock interactions between 120-180 ºC. On the other hand, samples from the Elk Creek Hot Springs and the Barron Hot

Springs areas show relatively lower temperatures (120-140 ºC) at depth.

Figure 7. Camas Prairie water samples plotted on a Giggenbach diagram.

Results of other conventional geothermometers for these areas are given in Table 1. Quartz (no steam loss), chalcedony, and Mg-

corrected Na-K-Ca geothermometers resulted in temperatures between 113 and 152 °C for the Magic Hot Springs area. Silica-enthalpy

mixing models were applied with compositions measured in water samples from the well and a well producing cooler groundwater. The

runoff channel sample was not considered because of the apparent heat loss. The chalcedony-enthalpy mixing model resulted in a

reservoir temperature of 145 °C with about 50% dilution. Similarly, the quartz-enthalpy mixing model resulted in a reservoir

temperature of 181 °C with about 60% dilution.

Samples from Elk Creek (HS1 and HS2) yield quartz (no steam loss), chalcedony, and Na-K-Mg reservoir temperatures between 86 ºC

and 115 °C, with values consistent between samples (Table 1). For samples from the Wardrop, Sheep, and Wolf Hot Springs areas

these geothermometers yield temperatures between 85 ºC and 124 ºC, while the chalcedony-enthalpy and quartz-enthalpy models yield

higher temperatures (133 °C and 173 °C, respectively). Similar results are obtained for the Barron Hot Springs area (Table 1).

5.2 Isotope geothermometry

For a few samples, we measured the oxygen isotope values in SO4 and water for δ18O (SMOW) sulfate-water isotope geothermometry.

The isotope geothermometric temperature estimate for the Magic Hot Springs RLW is found to be about 237 ºC. This temperature is

significantly hotter than the temperature estimates with conventional geothermometers. Moreover, other isotopic compositions for water

samples from this area also indicate a high-temperature water-rock interaction. The water isotope composition of the well water (δD = -

151‰, δ18O = -16.9‰, SMOW) is significantly shifted off the meteoric water line (the runoff sample was also shifted, but was also

clearly evaporated during cooling). In addition, the isotopic composition of dissolved methane in the water (δD = -203‰, SMOW and

δ13C = -22.0‰, Vienna-PDB) suggests typical of methane produced in a magmatic system (Conrad et al., 2016). The helium isotope

value for this system, Rc/Ra = 1.62, is high, and potentially indicates the presence of a magmatic component in these fluids (Dobson et

al., 2015).

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Table 1. Temperature estimates for Camas Prairie area water samples

Springs/wellsa RTEst

±σb GeoTc OHSO

18

24O

d Quartz (nsl)e Chalcedonyf Na-K-

Cag Silica-enthalpyh

Magic HS RLW 163±2 194±3 237 139 113 149 145 (181)

Magic HS RLWR 151±3 233 142 116 143

Elk Creek HS1 125±4 125±3 114 86 110

Elk Creek HS2 123±4 127±3 136 115 86 107

Wardrop HS 181±3 186±5 133 123 95 85

133 (173)

Hot Spring Ranch HS 1 188±3 187±5 126 98 89

Hot Spring Ranch HS 2 194±2 186±3 124 96 124

Hot Spring Ranch HS 3 188±1 188±4 124 96 101

Sheep HS 198±11 117 88 70

Wolf HS 203±2 183±3 114 85 124

Barron HS1 103±2 141±3 128 100 111

126 (162)

Barron HS2 104±6 143±3 128 100 115

Barron W2 419

Barron W2 104±4 129±7 127 99 94

Barron W3 103±5 129±6 114 85 106

Punkin Corner area W 105±6 124 96 86

Sun Valley Ranch W 108±4 124 96 88

a HS: Hot spring, WS: Warm spring, S: Spring, W: Well, RLW: Reservoir Landing well, RLWR: Reservoir Landing well runoff; b RTEst estimated temperature with associated standard error (Palmer et al., 2014; Mattson et al., 2015); c GeoT multicomponent

geothermometer (Spycher et al., 2014); d sulfate-water oxygen isotope geothermometer temperature (Fowler et al., 2013); e

quartz (no steam loss) geothermometer temperature (Fournier, 1977); f chalcedony geothermometer temperature (Fournier, 1977); g Mg-corrected (where applicable) Na-K-Ca geothermometer temperature (Fournier and Truesdell, 1973; Fournier and Potter II,

1979); h Temperature with silica-enthalpy mixing model (where applicable) using chalcedony solubility, temperature with quartz

solubility given in parenthesis

The Barron Well 1 from the Barron Hot Springs area resulted in an unusually high (419 ºC) with 18O sulfate-water isotope

geothermometry. This temperature estimate exceeds the effective range of this geothermometer, but potentially could indicate a high

temperature source for the sulfate. This could be supported by the δ34S of the sulfate, which at -8.3‰ (Vienna-Canyon Diablo) was by

far the lowest of any of the measured samples in these areas (Mattson et al., 2016). This is very low value for the sulfate usually

indicates that it is formed from oxidation of pyrite, suggesting it might be a reliable indicator of formation in a hydrothermal system.

However, it should be noted that SO4 concentrations measured in this well are 20 to 40 times greater (173-211 mg/L) than in nearby

wells (5-9 mg/L) Barron W2 and W3, which yield otherwise similar water compositions and geothermometry results. This suggests that

the high SO4 concentration in Well 1 may be related to the oxidation of shallow pyritic layers/zones that are unrelated to the present

hydrothermal activity in this area. However, it is notable that the Barron Well 1 provides the highest He Rc/Ra value (2.36) in the

Camas Prairie area. Earlier, Dobson et al. (2016) suggested that such a high He isotope signature in the Barron Hot Springs area might

be associated with deeper magmatic activity. Moreover, the Camas Prairie area, especially, the southern parts, has young volcanic

activity (the Pothole basalt) as reported by Garwood et al. (2014).

The samples from the Elk Creek Hot Springs and the Wardrop/Sheep/Wolf Hot Springs areas resulted in isotope geothermometric

temperatures in the range of 133 to 136 ºC. Unlike the Magic Hot Springs and Barron Hot Springs area, these northern Camas Prairie

areas exhibit lower He Rc/Ra values. For example, Elk Creek Hot Springs sample resulted in an Rc/Ra value of 0.85, which would

suggest an Idaho Batholith-type geothermal system for this area (Rc/Ra values ranging from 0.22-0.95, Dobson et al., 2015). In reality,

the Elk Creek Hot Springs are located further north from the proper Camas Prairie area in the Idaho Batholith region (Figure 4). Rc/Ra

values for the Wardrop Hot Springs and Wolf Hot Springs, which are located within the Camas Prairie area proper, are slightly higher

(1.13-1.33) which may either reflect the Idaho Batholith signature or could represent a mixed signature of the Idaho Batholith and the

Barron Hot Springs area type settings.

5.3 Multicomponent geothermometry

The multicomponent geothermometry software tools RTEst and GeoT were applied to several samples from the Camas Prairie area.

These tools make use of full chemical analyses to imply deep reservoir temperatures on the basis of computed saturation indices of

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selected minerals expected (or known) to be part of a deep reservoir mineral assemblage. In addition, both tools make use of numerical

optimization to reconstruct the composition of deep thermal fluids to compensate for mixing/dilution and degassing effects that can

impact geothermal fluids as they ascend to ground surface. For more information on these tools, we refer readers to Palmer et al. (2014)

and Spycher et al. (2014). Although both RTEst and GeoT are based on essentially the same concepts for temperature determinations, it

is important to note that they rely on different thermodynamic databases, computational routines, and optimization schemes. Here both

tools were applied independently, by different users and purposely without knowledge of each other’s mineral selections for temperature

determinations. Such “blind” comparison was preferred to a true benchmarking exercise to assess result variability when applying this

type of geothermometry approach.

For the Wardrop/Sheep/Wolf Hot Springs area samples, RTEst was applied with a mineral assemblage consisting of analcime, calcite,

chalcedony, fluorite, K-clinoptilolite, and K-feldspar. All RTEst runs were performed with three optimization parameters (mass of

water, fugacity of CO2 and temperature), and Al and Mg concentrations were taken as measured values. The RTEst reservoir

temperature estimates for these hot springs range from 181-203 °C with a large (up to 4) dilution factor (Figure 8a). The GeoT

temperature determinations for the Wardrop Hot Springs samples were based on a mineral assemblage consisting of low albite, Ca-

beidellite, calcite, clinoclore, laumontite, Mg-montmorillonite, and quartz. In this case, the concentrations of Mg and Al were optimized

(to compensate for re-equilibration effects upon cooling) together with the dilution factor (3.1) and fraction of degassing (assuming

100% CO2 in the non-condensable gas phase); in addition, best results (in terms of mineral clustering) were obtained assuming reaction

of calcite along the fluid cooling path. This approach resulted in a temperature estimation of 185 ºC. A similar GeoT temperature of

about 183 ºC was obtained for Wolf Hot Springs using the same approach and similar mineral assemblage (Table 1, Figure 8b).

Figure 8. Computed plots of mineral saturation indices (Log (Q/K)) for reconstructed Wolf Hot Spring fluids with: a) RTEst

(ana: analcime, cal: calcite, cli: K-clinoptilolite, flu: fluorite, kfs: K-feldspar, and qtz: quartz), indicating a likely

reservoir temperature of about 203±2 ºC; b) GeoT, showing a likely reservoir temperature of about 183±3 ºC (including

optimization Al and Mg concentrations).

RTEst was applied to the Elk Creek Hot Springs (1 and 2) water samples using a mineral assemblage consisting of calcite, chalcedony,

K-clinoptilolite, K-feldspar, Na-beidellite, and paragonite with three optimization parameters (mass of water, fugacity of CO2 and

temperature). GeoT was applied to these samples as described above and with a mineral assemblage consisting of albite-low, calcite,

chalcedony, clinochlore, laumontite, microcline, Na-beidellite, and Na-montmorillonite. Both the RTEst and GeoT yielded the same

value of 125 ºC as reservoir temperature.

RTEst was used with the Magic Hot Springs area samples using a local groundwater composition to optimize the mass of dilution water

and solute concentrations along with two other optimization parameters - fugacity of CO2 and temperature. A mineral assemblage

consisting of chalcedony, dolomite, K-clinoptilolite, K-feldspar, and Mg-beidellite was used to make temperature determinations,

yielding a calculated reservoir temperature of about 163 °C for this area (Table 1). Similar to the estimated degree of dilution derived

from the chalcedony-enthalpy mixing model, the RTEst modeling indicates that the Magic Hot Springs geothermal water is diluted by 2

times with local groundwater. Using two different mineral assemblages, GeoT yields higher calculated temperatures: 194 ºC and a

dilution factor of 3.2, when considering calcite, enstatite, microcline, Mg-montmorillonite, heulandite, low albite, and chalcedony; and

197 ºC and a dilution factor of 2.4 with calcite, clinochlore, microcline, quartz, kaolinite, low albite, and heulandite.

RTEst was applied to the Barron Hot Springs area samples using a mineral assemblage consisting of calcite, chalcedony, fluorite, K-

clinoptilolite, K-feldspar, Na-beidellite. The reservoir temperature estimates with three optimization parameters (mass of water, fugacity

of CO2 and temperature) RTEst runs for hot springs and wells of this area resulted in lower reservoir temperatures, ranging from 79 ºC

to 108 °C (Table 1). The RTEst results also indicated that the thermal water samples from this area have a negligible dilution, which

could imply that the features in this area are taping water from a low-temperature re-equilibrated reservoir. In contrast, GeoT runs for

the same samples, using a mineral assemblage of low albite, calcite, clinoclore, laumontite, microcline, Na-beidellite, Na-

Neupane et al.

10

montmorillonite, and quartz resulted in higher reservoir temperatures (129-143 ºC) (Table 1) and a dilution factor ranging 1.2 - 1.9

(dilution factor = 1 for no dilution).

The chemical compositions of thermal waters from the northern (the Wardrop/Sheep/Wolf Hot Springs areas) and central-southern parts

(Barron Hot Springs area) of the Camas Prairie (excluding Magic Hot Springs and the Elk Creek Hot Springs area, both of these areas

also have different water compositions) have different water compositions which may indicate different hydrogeological and

geochemical settings at depth. Geologically, the northern Camas Prairie basin reportedly has less basalt at depth, whereas the southern

Camas Prairie basin is known to have more basalt layers (Glen et al., 2017). In general, both surface and subsurface waters of the Camas

Prairie area flow from west to east towards the Big Wood River, and ultimately, to the Magic Hot Springs Reservoir (Wallace, 1972).

Precipitation falling in the Soldier Mountains to the north provides the majority of recharge water to the Camas Prairie

groundwater/geothermal systems. The thermal water that moves up along the range-forming faults near/along the northern boundary of

the Prairie is significantly diluted with pristine water that several south flowing creeks from the mountains bring into the Prairie. On the

other hand, the Mount Bennett Hills to the south offers only a minor recharge to the Camas Prairie. Unlike the northern part of the

Prairie, the groundwater/geothermal aquifers in the southern parts of the Prairie are likely to have longer residence times (Wallace,

1972). Although a long residence time results in higher field temperatures [the highest temperature of 73 ºC in the area was recorded for

Barron Hot Spring II (Mitchell, 1976), however, this feature is no longer active], it also helps re-equilibrate the thermal water at lower

temperatures. Because of the structural and hydrogeological controls, the hot springs in the northern parts of the Camas Prairie are

issuing diluted thermal waters whereas the sampling features in the southern part are likely issuing re-equilibrated water.

6. CONCLUSIONS

In this study, we combined geochemical and isotopic compositions of water samples from several hot springs and wells (with elevated

temperatures) to identify and evaluate the potential geothermal areas in the Camas Prairie in Idaho. All known and accessible sampling

features in the area were sampled over the three year period (2014-2016). Specifically, we identified four geothermally active areas in

Camas Prairie area. These areas from east to west (and south) are the Magic Hot Springs area, the Elk Creek Hot Springs area, the

Wardrop/Sheep/Wolf Hot Springs areas, and Barron Hot Springs area. Although the majority of all thermal features in Camas Prairie

area produce Na-HCO3 type water with likely meteoric origin, the water samples from the four potential geothermal areas can be

characterized by their unique geochemical signatures. The Magic Hot Springs area (Magic Hot Springs RLW) water samples have

relatively higher level of TDS and silica, near neutral pH, higher levels of Na, and HCO3 with significant amount of Cl and SO4. The

Elk Creek Hot Springs issue alkaline (pH>9), high F (15 mg/L), Na-HCO3 type waters with very low Mg but significant Cl and SO4.

The water samples of the Wardrop/Sheep/Wolf Hot Springs have high pH (9.0-9.9), relatively low level of TDS (220-350 mg/L), low

concentrations of Ca and Mg, and intermediate concentration of F (1.9-3.7 mg/L). Finally, the Barron Hot Springs area water samples

are near-neutral to slightly alkaline (pH 7.4-8.5) with slightly higher level of TDS (380-640 mg/L), low concentration of Mg, and

moderately high concentrations of F (7-13 mg/L). The groundwater/cold springs water samples from this area are of mostly Ca-HCO3

type water with near-neutral pH, low levels of F, Na, and wide range of Ca.

The δD and δ18O values of the Camas Prairie area samples plot close to the local meteoric water line; however, a few samples have

oxygen isotope composition shifted 1-3‰ to the right of the meteoric water line (such as, the Barron Well 1, Elk Creek Hot Spring 2,

Magic Hot Springs RLW). These isotopic compositions, in general, indicate that the groundwater/hydrothermal waters in the area are of

meteoric in origin. The He Rc/Ra values of some samples from this area are found to be elevated. For example, the Magic Hot Springs

RLW and the Barron Well 1 have Rc/Ra values of 1.62 and 2.36, respectively, and these values may potentially indicate a magmatic

component in these fluids that may contribute to elevated heat flow in the area. However, the Elk Creek Hot Springs and the

Wardrop/Wolf Hot Springs samples show relatively lower (0.85 and 1.13-1.33, respectively) Rc/Ra values and potentially, an Idaho

Batholith-type geothermal setting.

Solute geothermometry results indicate promising temperature estimates for the Magic Hot Springs area and Wardrop/Sheep/Wolf Hot

Springs area where the temperature estimates could be as high (or higher than) as 160 ºC and 200 ºC, respectively. For the Elk Creek

Hot Springs and Barron Hot Springs area, the upper temperature estimates are close to 140 ºC.

In general, our geochemical/isotopic study shows that the Camas Prairie area could potentially host resources with temperatures as high

as 200 ºC. Our results suggest that the area might have two potential geothermal resource types: one associated with the Idaho Batholith

to the north, and a second related to elevated heat flow associated with Quaternary volcanism and intrusions.

ACKNOWLEDGEMENTS

This work was supported by funding by the Assistant Secretary for Energy Efficiency and Renewable Energy, Geothermal Technologies

Office of the U.S. Department of Energy under the U.S. Department of Energy Contract Numbers DE-AC07-05ID14517 for Idaho

National Laboratory, DE-AC02-05CH11231 for Lawrence Berkeley National Laboratory, and DE-EE0006733 for Utah State

University. We thank landowners who generously provided access to sampling locations. We also thank Mr. Lee Barron and Dr. Roy

Mink for their assistance in coordinating with landowners and field work during the summer 2016 sampling campaign. Ms. Debbie

Lacroix [University of Idaho/Center for Advanced Energy Studies (CAES)] conducted chemical analyses of the water samples.

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